Dual-Phase steels, the structure of which comprises martensite, and possibly some bainite, in a ferritic matrix, have become widely used because they combine a high strength with high deformation capacity. As delivered, their yield strength is relatively low compared with their fracture strength, which gives them a very favorable yield strength/strength ratio during forming operations. Their work-hardening ability is very high, which allows good deformation distribution in a collision and produces a much higher yield strength in a part after forming. Thus, parts as complicated as those produced with conventional steels can be made, but with better mechanical properties, which enables a reduction in thickness to meet the same functional specification. In that way, these steels are an effective answer to the requirements of vehicle lightening and safety. In the field of hot-rolled (with a thickness for example of 1 to 10 mm) or cold-rolled (thickness for example of 0.5 to 3 mm) sheets, this type of steel especially finds applications for structural and safety parts for motor vehicles, such as crossmembers, side members, reinforcing parts, or even pressed steel wheels.
Modern requirements for lightening and the reduction of energy consumption have resulted in an increased demand for very high-strength dual-phase steels, that is to say of which the mechanical strength Rm is between 980 and 1100 MPa. In addition to this level of strength, these steels must have good weldability and good continuous hot-dip galvanizing capacity. These steels must also have good bending capacity.
The manufacture of high-strength Dual-Phase steels is for example described in the document EP 1201780 A1 relating to steels having the composition: 0.01-0.3% C, 0.01-2% Si, 0.05-3% Mn, <0.1% P, <0.01%8, and 0.005-1% Al, of which the mechanical strength is greater than 540 MPa, which have good fatigue strength and hole expansion ratio. However, most of the examples presented in this document exhibit strength less than 875 MPa. The rare examples in this document going beyond this value relate to steels with a high carbon content (0.25 or 0.31%) of which the weldability and the whole expansion ratio are not satisfactory.
The document EP 0796928 A1 also describes cold-rolled Dual-Phase steels of which the strength is greater than 550 MPa, having the composition 0.05-0.3% C, 0.8-3% Mn, 0.4-2.5% Al, and 0.01-0.2% Si. The ferritic matrix contains martensite, bainite and/or retained austenite. The examples presented show that the strength does not exceed 660 MPa, even with high carbon content (0.20-0.21%).
The document JP 11350038 describes Dual-Phase steels of which the strength is greater than 980 MPa, having the composition 0.10-0.15% C, 0.8-1.5% Si, 1.5-2.0% Mn, 0.01-0.05% P, less than 0.005%8, 0.01-0.07% Al in solution, and less than 0.01% N, also containing one or more of the following elements: 0.001-0.02% Nb, 0.001c0.02% V, 0.001-0.02% Ti. This high strength is obtained however at the expense of a large addition of silicon which of course allows martensite to form, but can nevertheless result in the formation of surface oxides which negatively affect the dip coatability.